In mobile communications, typified by mobile phones, receptions of RF (radio frequency) signals from base stations are performed. RF signal down-conversions, which are receiving methods suitable for receivers of mobile phones, are classified into a single-stage type method and a multi-stage type method.
<<Super Heterodyne Receiver>>
Super heterodyne receivers of a multi-stage type have a large number of passive elements, and therefore they are not suitable for integration.
<<Direct Down-conversion Receiver>>
In direct down-conversion receivers of a single-stage type, an orthogonal RF modulator converts RF received signals into DC components directly. Direct down-conversion receivers have an advantage that the level of integration is higher in comparison to super heterodyne receivers.
<<Low IF Receiver>>
In low IF receivers, which are similar to direct down-conversion receivers, RF received signals are converted down into not DC frequencies but intermediate frequencies of hundreds of kilohertz through orthogonal RF down-conversion. The image-rejection filter can be integrated and the merit of direct down-conversion is still retained. However, they require a high performance A/D converter for image rejection. After analog-to-digital conversion, digital signals from the A/D converter undergo down-conversion into digital DC components before digital filtering.
<<Digital IF Receiver>>
In a digital IF receiver, an RF modulator performs first down-conversion of an RF received signal into a first intermediate frequency signal, as described in Non-patent Document 1, Teemu O. salo et al, “80-MHz Band pass ΔΣ Modulators for Multimode Digital IF Receivers”, IEEE JOURNAL OF SOLID-STATES CIRCUITS, VOL. 38, NO. 3, MARCH 2003, PP. 464-474. The first intermediate frequency signal is subjected to analog-to-digital conversion by a band pass ΔΣ modulator, which performs high-resolution analog-to-digital conversion of a narrow-band high frequency signal, and thus converted into a digital signal. Through digital signal processing, a software program implements different functions. On this account, a digital IF receiver is flexible. In addition, as a digital IF receiver performs orthogonal mixing and filtering for selection of a channel in digital regions, it allows us to reap the benefit of miniaturization. From this point of view, a digital IF receiver is also flexible. As a band pass ΔΣ modulator conducts high-resolution analog-to-digital (A/D) conversion of narrow-band high frequency signals, it is one of the most possible tools for direct IF digitalization.
<<ΔΣ Modulator>>
In a ΔΣ modulator using an over sampling ratio that a sampling frequency much higher than a Nyquist frequency is utilized, quantization noise spectrums are shaved by a feedback loop. Therefore, in comparison to a traditional Nyquist ratio A/D converter, an over sampling type ΔΣ modulator is less sensitive to nonideal properties of an analog circuit. In a low-pass (LP) ΔΣ modulator with an integrator used as a loop filter, quantization noises are reduced at a DC frequency greatly.
When a resonator is used as a loop filter instead of an integrator, a band pass (BP) ΔΣ modulator is constructed. This is as described in Non-patent Document 2, Teemu Salo et al, “A LOW-VOLTAGE SINGLE-OPAMP 4TH-ORDER BAND PASS ΔΣ MODULATOR”, The 2001 IEEE International Symposium on Circuits and Systems, 6-9 May 2001, PP. 352-355. A resonator of a discrete-time type is constituted by two delay devices or integrators connected in series, which have a loop feedback. This resonator includes a switched capacitor (SC) driven by two-phase clocks.
In addition, there is the description on a DSP-based digital IF AM/FM car-radio receiver which includes an A/D converter constructed with a band pass ΔΣ modulator; DSP is flexible, small in chip area, low in power consumption, and superior in performance of signal processing. This is described in Non-patent Document 3, F. Adduci et al, “A DSP-Based Digital IF AM/FM Car-Radio Receiver”, Proceedings of the 29th European Solid-State Circuits Conference 16-18 Sep. 2003, PP. 201-204.
Further, in Non-patent Document 4, J. Silva et al, “Wideband low-distortion delta-sigma ADC topology”, ELECTRONICS LETTERS, 7 Jun. 2001 Vol. 37, No. 12, PP. 737-738, it is described that in a low-pass (LP) ΔΣ modulator with an integrator used as a loop filter, an analog input signal is directly supplied to an input of a quantizer, whereby the integrator is allowed to handle only quantization noise, and low distortion is achieved.
Also, in Non-patent Document 5, KiYoung Nam et al, “A 1.2-V 15-bit 2.5-MS/s Oversampling ADC with Reduced Integrator Swings”, IEEE 2004 CUSTOM INTEGRATED CIRCUITS CONFERENCE, PP. 515-518, it is described that in a low-pass (LP) ΔΣ modulator with an integrator used as a loop filter, an analog input signal is directly supplied to an input of a quantizer, thereby reducing the amplitude of the integrator and suppressing the nonlinearity of the integrator.
Still further, Non-patent Document 6, R. Maurino at al. “MULTIBIT QUADRATURE SIGMA-DELTA MODULATOR WITH DEM SCHEME”, Proceedings of the 2004 International Symposium on Circuits and Systems, 6-9 May 2004, PP. 1136-1139, contains the description that in a band pass (BP) ΔΣ modulator of a quadrature type, an analog input signal is directly supplied to an input of a quantizer. The quadrature band pass (BP) ΔΣ modulator described in Non-patent Document 6 produces complex orthogonal digital output signals from complex orthogonal analog input signals composed of real and imaginary parts. This is as described in Non-patent Document 7, Stephen A. Jantzi et al, “Quadrature Bandpass ΔΣ Modulation for Digital Radio”, IEEE JOURNAL OF SOLID-STATES CIRCUITS, VOL. 32, NO. 12, DECEMBER 1997, PP. 1935-1950.